Implications of Site-specific Mass Absorption Cross-section (MAC) to Black Carbon Observations at a High-altitude Site in the Central Himalaya

Accurate estimation of black carbon (BC) from the widely used optical attenuation technique is important for the reliable assessment of their climatic impact. The optical instruments use Mass Absorption Cross-section (MAC) for converting light attenuation records to BC mass concentrations and Aethalometer is a widely used optical instrument for BC estimation. Several studies have shown large variability in MAC values. It is thus necessary to examine the accuracy and consistency of MAC values obtained using Aethalometer over distinct geographic locations and seasons. In the present study, MAC values are derived using simultaneous observations (2014-2017) from an EC-OC analyzer and an Aethalometer (AE-42) over a high altitude central Himalayan site at Nainital (29.4(o)N, 79.5(o)E, 1958 a.m.s.l). The observations reveal that the annual mean value of MAC (5.03 +/- 0.03 m(2)g(- 1) at 880nm) is significantly lower than the constant value used by the manufacturer (16.6 m(2)g(- 1) at 880nm). The estimated MAC values also showed significant seasonal variation, spanning over a range from 3.7 to 6.6 m(2)g(- 1). It is found that the seasonal variability of elemental carbon (EC), air mass variation and meteorological parameters play an important role in the changes in MAC values over this region. Multi-wavelength determination of MAC shows the contribution of absorption by species other than EC at shorter wavelengths. MAC does not show a clear diurnal variation, unlike EC and absorption coefficient. The slope of EC vs. corrected equivalent black carbon (eBC) showed a significant improvement during all seasons when compared with uncorrected eBC. This lends credibility to the fact that the use of site-specific MAC leads to more reliable estimates of eBC over the central Himalayan region. It is found that, instead of using the site specific MAC value, had we used the one supplied by the instrument, we would have underestimated the radiative forcing by about 7.8Wm(- 2) which amounts to a reduction by 24 %.